Many commercially popular wireless communication radio access network technologies operate in a hub and spoke model. In the hub and spoke model an access point, which is typically connected to the core network, provides communication to terminals connected to the access point. Traditionally each access point operates very independently of the other access points, treating the terminals associated with the other access points as interference. Multiple input multiple output (“MIMO”) technologies have demonstrated that utilizing more antennas to process the terminals is advantageous. Further advancements show that centralizing multiple access points and treating it like a larger MIMO system by jointly processing across the antennas of multiple access points is further advantageous. Research literature calls this technique cooperative MIMO, network MIMO, distributed MIMO, and/or virtual MIMO, among other names. Two popular wireless networks that are moving in this direction are Wi-Fi and cellular networks. The detailed description herein will be in the context of a cellular network, but is not intended to exclude application to other related technologies, such as Wi-Fi.
In the Long Term Evolution (“LTE”) specification, Release 11, the concept of coordinated multipoint (“CoMP”) was first introduced. CoMP allows for multiple cellular sites to coordinate their transmission or reception in order to minimize effective interference and increase performance. Although technically possible to use these techniques in the uplink prior to release 11, it indicated a movement to more interconnection and coordination of multiple sites.
There is also a trend towards centralizing processing of many sites in Centralized RANs (“C-RAN”). In the case of C-RAN, Remote Radio Heads (“RRHs”) are placed at the antenna sites where the radio frequency (“RF”) signal is digitized and transmitted, typically over fiber, to a central location for processing. The C-RAN architecture provides many benefits since the equipment is centralized, such as reducing site-leasing costs, reducing maintenance, increased deployment ease, and better energy efficiency. From a technology perspective, the Physical (“PHY”) layer and Media Access Control (“MAC”) layer can be enhanced to provide better interference management or suppression, among other advantages.
CoMP is related to C-RAN in that it is the standardization of the PHY/MAC technology that can leverage the C-RAN architecture. There is no actual requirement for centralization though. In fact, the only requirement is that the interfaces between sites be fast enough to support the technology. Therefore, although C-RAN is a convenient term, it is not meant to be restrictive as to the distribution of processing between the sites.
For clarification, terminology used herein is defined below using exemplary cellular-based terminology. Such terminology use is not intended to limit the subject matter described to cellular applications.                Base Station—Equipment performing the processing of antenna data. This may be located at a cellular site or at a remote site, such as in a C-RAN.        Distributed—A term used to indicate a base station that has processing independent of other sites, i.e., not C-RAN or CoMP. Specifically, no information is shared in the PHY layer between that base station and other ones.        Site—One or more antennas from one physical location. “Site” can either refer to the location or the processing of the data from that location.        Cluster—In the conventional C-RAN model, a grouping of sites and/or sectors including antennas that provide antenna data that is processed centrally.        Sector—Some portion of a site served by one or more antennas. A site may have multiple sectors, particularly when using directional antennas. A common deployment is three sectors per site. There can be as few as one sector per site, such as when omnidirectional antennas are utilized. In such cases, the terms “site” and “sector” can be used interchangeably.        
In the description that follows, in most cases there is an assumption for simplicity that an omnidirectional antenna is used at a site, which results in there being one sector per site. The term “sector” is used herein but under such an arrangement the term “site” can also be used. Note that it is not intended that the subject matter described herein be limited to single sector sites.
Going a step further into the design of a traditional distributed base station architecture, without intercommunication between multiple sites or sectors, the PHY layer is then by definition also being processed in a distributed way. Distributed PHY means that the PHY layer processing is only calculated over the antennas associated with each individual sector. Typically, this means that, from the uplink perspective, only the users associated with the same sector are being processed and all other users outside of that sector are treated as interference as discussed above. The dual is also true on the downlink where the other sectors create interference at the user terminal.
As some cellular architectures have progressed beyond the distributed base stations, such as with CoMP and C-RAN, the PHY layer is able to be adapted in conjunction. Although not necessary by design, with the centralization or coordination of multiple sectors or sites, the physical layer can be enhanced to increase the performance of the network. For the uplink, treating the system as a larger MIMO receiver and jointly processing over the antennas of multiple sectors will typically improve performance.
Based on most MIMO receiver designs, such as those employing linear least square (“LS”) or linear minimum mean square error (“MMSE”) equalizers, channel estimates are required for the users being modeled in them. Therefore, channel estimation is conventionally performed for the users associated with the sectors utilized for joint processing. This approach can be viewed as a clustered CoMP or C-RAN.
FIG. 4 is a block diagram illustrating an exemplary wireless communication system in accordance with the prior art. FIG. 4 shows an example C-RAN 400. The C-RAN 400 has 12 sites, Sites 1-12, where each site is a single sector covering the area indicated by the hexagonal pattern for that site, with the exception of Site 9, which includes three sectors strictly for illustrating that a site can have multiple sectors. In this example, each of the 12 sites is shown with a corresponding antenna 401-412, respectively, attached to remote radio heads which bring that antenna data back to a central C-RAN processing point where all of the sites are processed. While only a single sector per site and only a single antenna per sector is shown in Sites 1-8 and 10-12 for the simplest case, it will be understood that a site can have multiple sectors and a sector can have multiple antennas. Note that in the case of Site 9, there are three sectors 409A-409C, with each being served by one or more directional antennas.
FIG. 4 shows an example of a clustered C-RAN system in which there are 4 clusters, Clusters C1-C4, indicated by darker borders, where Sites 1-3 make up C1, Sites 4-6 make up C2, Sites 6-9 make up C3, and Sites 10-12 make up C4. The sites are clustered this way for clustered C-RAN processing. For example, Sites 1-3 of Cluster C1 are processed jointly by utilizing a MIMO equalizer across the antennas of the sites. Since the MIMO equalizer traditionally relies on channel estimates from each antenna to each user, channel estimation is conventionally performed from all of the antennas associated with Sites 1-3 to all of the users in Sites 1-3. In this case, user equipment (“UE”) for users U1, U2, and U3 are in Sites 1, 2, and 3, respectively, and are all jointly processed and modeled for channel estimation in Cluster C1 based on antenna data from antennas 401-403 in Sites 1-3.
Although clustering is effective in increasing performance, significant degradation in system performance still exists from interference. In a sense, it can be viewed as trading off interference from edge of cell users in a distributed system to edge of cluster users in a centralized system. For example, in FIG. 4, users U4 and U5 in Sites 4 and 5 would interfere with the joint processing of Cluster C1, resulting in degradation of system performance.
Accordingly, there exists a need for methods, systems, and computer program products for jointly processing multiple sectors in a wireless communication network.